1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal color-display device, and more particularly to such display device capable of cell gap compensation particularly in the peripheral area.
2. Related Background Art
For use in a liquid crystal display device, there is already known a dyed color filter obtained by forming a matrix layer of a hydrophilic substance such as gelatin, casein or polyvinyl alcohol on a substrate and dyeing said matrix layer with suitable dyes.
Such dyeing method can relatively easily satisfy the spectral characteristics required for the color filter because of a large number of usable dyes, but it is associated with a low production yield because it employs a poorly controllable wet process, for dyeing the matrix layer, by immersing said matrix layer in a dyeing bath containing the dye, and also because it involves complex process steps of forming a dye-preventive intermediate layer for each color. Also this method is not adequate in case the filter is subjected to a thermal treatment because the thermal stability of the usable dyes is relatively low and is generally limited to 150.degree. C. or lower, and the matrix layer itself lacks the reliability in thermal and light resistances.
On the other hand, there is also known a color filter employing colored resin, composed of certain coloring substances dispersed in transparent resin.
For example, in a color filter employing colored resin films composed of polyamide resin containing coloring substances, such as disclosed in the Japanese Laid-Open Patents Sho 58-46325, Sho 60-78401, Sho 60-184202, Sho 60-184203, Sho 60-184204 and Sho 60-184205, the polyamide resin itself has excellent heat and light resistances, but the formation of pattern of the color filter has to rely on a printing process which is inadequate for fine pattern, or a complex process of forming a photoresist mask on the colored resin film and etching said film through said mask, because said resin is non-photosensitive.
On the other hand, a color filter employing color resin films containing coloring substances in photosensitive resin, as disclosed in the Japanese Laid-Open Patents Sho 57-16407, Sho 57-74707 and Sho 60-129707, can simplify the process as fine patterns of the color filter can be formed by ordinary photolithographic steps.
However, in the preparation of a color filter with such colored resins containing coloring substances in the photosensitive resin, the exposure energy required for photohardening of said photosensitive resin becomes considerably larger than in the usual cases, because such coloring substances generally have light absorption in the exposure wavelength region.
In general, the exposure light intensity in photosensitive resin decreases exponentially with the depth from the surface receiving the exposing light. Presence of the light-absorbing coloring substance in the photosensitive resin enhances the attenuation of exposing light intensity with the depth, thus eventually causing insufficient photo-hardening of the photosensitive colored resin film at the bottom thereof, namely at the interface with the substrate, and resulting peeling of said film at the pattern development.
On the other hand, an increase of the exposure energy for compensating the attenuation of the exposing light energy may lead to deterioration of the colored resin itself.
Also the presence of the coloring substance in the colored resin film induces surface coarseness thereof, which not only causes light scattering on the surface of the color filter, thereby deteriorating the optical performance thereof, but also may disturb the orientation of the liquid crystal molecules in case said color filter is positioned on the internal surface of the liquid crystal display device.
As explained above, the formation of a color filter with colored resin containing coloring substances in photosensitive resin still involves drawbacks in the stability and durability of the color filter films in the process of formation thereof and in the performance of the obtained color filter.
In order to overcome the above-mentioned drawbacks, the present applicant already proposed, in the Japanese Patent Application sho 62-22461 (Japanese Laid-Open Patent Sho 63-191104), a color filter which enables effective pattern formation with a reduced exposure energy, prevents the deterioration of the colored material at the exposure, reduces the difference in photo-hardening in the direction of depth of the film, and provides a smoother surface on the colored resin film. This color filter is provided with plural patterned and colored resin layers which are formed by employing colored resin containing at least coloring substance in photosensitive resin and by repeating photolithographic steps, wherein the amount of dispersed coloring substance is gradually increased from the surface to the bottom of the colored resin layer.
A liquid crystal color-display panel is formed by positioning, in mutually opposed relationship, a substrate which bears thereon a color filter, formed by one of the above-mentioned methods, and a protective passivation layer and on which pixel units are formed by surface electrodes and metal lead electrodes, and another substrate on which pixel units are formed by surface electrodes and metal lead electrodes. For obtaining a uniform cell gap in such display panel, there is principally employed a method of scattering gap-retaining material in the area of pixel units and printing sealant, with gap-retaining material of a same diameter, in the peripheral area of the substrate, before both substrates are mutually adhered.
Also for obtaining a more uniform cell gap, there is known a method of forming the color filter not only in the pixel area but also in the peripheral area and providing the gap-retaining material of a same diameter in both areas.
The cell gap of the color display panel employing ferroelectric liquid crystal is as small as 1-2 .mu.m, which is significantly smaller than that of the display panels employing super-twisted nematic or other liquid crystal, and has to be maintained at a uniformity of 0.05-0.30 .mu.m. However, in the above-mentioned first conventional structure employing the gap-retaining material in the pixel area and the gap-retaining material of a same diameter mixed in the sealant printed in the peripheral area, the glass substrates may generate significant bending between the sealed peripheral area and the pixel area. Particularly, in the end portions of the pixel area, the liquid crystal layer becomes thinner than in the central portion, whereby the chevron structure of liquid crystal molecules constituting the oriented state may be destructed and the orientation of said molecules may be disrupted.
On the other hand, in the above-mentioned latter conventional structure, in which the color filter is extended to the peripheral area and the gap-retaining material of a same diameter is provided both in the pixel and peripheral areas, the sealant printed on the color filter may show insufficient adhesion, because of the relatively poor adhesive character of the color filter, whereby the mutual alignment of the substrates, to be maintained by said sealant, may become difficult to maintain.